Cloud pricing is subscription based. You are eligible for support and automatic version updates as long as your subscription is active. When your subscription renews each month, you are automatically billed for host products and apps based on the number of users in your instance.
Each tier has a different base price. 1-100 users has a monthly fee of $0.50 per user, and then the price per user decreases as you increase your user tier.
For example, the price for 252 users would be calculated as 100 * $0.50 per user (1-100 users) + 100 * $0.42 per user (101-200 users) + 52 * $0.25 per user (250+ users) = $105 per month.